Telemetry system for drill bore holes

ABSTRACT

Apparatus and method for signaling within a drill bore hole without disturbing an ongoing drilling operation. Repeater stations are utilized at intervals along a drill string with the repeater input and output being in different noninterfering first and second modes. Furthermore, means are shown for causing successive repeater transmitters of any given mode to produce only negative feedback and hence to cause the overall system to be self-stabilizing. Special means are also shown for selectively attenuating one or both modes of transmission between repeater stations thereby substantially eliminating undesired feedback in the cases where self-stabilization cannot be guaranteed. Both active and passive attenuators are discussed as well as two exemplary overall systems. One exemplary system uses electric current for one mode and magnetic flux for the other mode while the other exemplary system utilizes different signal frequencies for the different transmission modes.

United States Patent [191 Still 1 Feb. 19, 1974 TELEMETRY SYSTEM FOR DRILL BORE HOLES [76] Inventor: William L. Still, RFD No. 2,-Box 73,

Purcellville, Va. 22132 [22] Filed: Mar. 31, 1971 [21] Appl. No.: 129,665

[56] References Cited UNITED STATES PATENTS 3,090,031 5/1963 Lord 340/18 LD 1,874,865 8/1932 Beverage 325/1 2,017,126 10/1935 Kroger 325/9 3,611,139 10/1971 George 325/9 3,079,549 2/1963 Martin 340/18 LD Primary ExaminerMaynard R. Wilbur Assistant Examinerl'l. A. Birmiel Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT Apparatus and method for signaling within a drill bore hole without disturbing an ongoing drilling operation. Repeater stations are utilized at intervals along a drill string with the repeater input and output being in different noninterfering first and second modes. Furthermore, means are shown for causing successive repeater transmitters of any given mode to produce only negative feedback and hence to cause the overall system to be self-stabilizing. Special means are also shown for selectively attenuating one or both modes of transmission between repeater stations thereby substantially eliminating undesired feedback in the cases where self-stabilization cannot be guaranteed. Both active and passive attenuators are discussed as well as two exemplary overall systems. One exemplary system uses electric current for one mode and magnetic flux for the other mode while the other exemplary system utilizes different signal frequencies for the different transmission modes.

41 Claims, 8 Drawing Figures PATENIED FEB .1 9 I974 SHEET 1. [IF 3 merz METB HE/LL INVENTOIR MAWMJ 527.44

ATTORNEYS SHEET 2 If 3 INVEN'I'OR TELEMETRY SYSTEM FOR DRILL BORE HOLES This invention generally relates to methods and apparatus for signaling or information transmission within a drill hole. It is applicable to any task which includes the requirement to transmit information from any point within the drill hole to the surface; from the drill hole to any point, or plurality of points, within the drill hole; or between any plurality of points within the drill hole. It is particularly applicable to the transmission of data while drilling is in progress, from sensors located within the drill hole, and attached to the drill string.

A number of techniques and devices have been proposed for achieving the transfer of data from within the drill hole to the surface, without the use of separate conducting cables. For example, Silverman in U.S. Pat. No. 2,354,887 proposes the use of a toroid which surrounds the drill pipe. Alternating current introduced into winding of the toroid produces a magnetic flux in the core of the toroid which, in turn, produces a current in the drill stem. The drill stem acts as a one turn loop through the core of the toroid with the earth acting as a return path to complete a current loop between points on the drill stem above and below the toroidp A variety of techniques are used at the surface to measure the drill stem currents thus produced. One of these techniques includes a second toroid surrounding the drill string at the surface. A portion of the current introduced into the drill string also flows through this second toroid and thereby produces a measureable voltage at the output terminals of the second toroid as will be apparent to those in the art.

in US. Pat. No. 2,411,696, Silverman proposes essentially the same system except that he uses an intermediate repeater station also attached to the drill stem. Here, he employs a second toroid to receive the data transmitted from a first toroid on a modulated carrier. He demodulates this data at the second toroid location and uses the demodulated information to remodulate a second carrier frequency which is then amplified and retransmitted, either through a third toroid, or through another winding on the second toroid.-

In US. Pat. No. 3,079,549, Martin proposes a series of repeater stations, operating on essentially the same frequency. In a'form of time duration frequency shift keying, all repeater stations receive and retransmit using toroids on the drill stem, as in Silvermans system. However, all repeater stations are only conditionally stable oscillators, while the master station, which Originates the data transmission operates with stable oscllators. The presumption is that the conditionally stable oscillators will tend to lock to the frequency controlled by the master station and thus, in time after stabilizing, the information will be relayed along the drill stem.

These prior art systems suffer from the common failing of inherently providing insufficient bandwidth for either high speed data transfer, or for multi-channel operation, wherein data from a number of sensors is to be transferred simultaneously. Furthermore, extensions of the technique, based on these teachings, to establish any major increase in data transmission rates, will soon meet inherent limits established by the technique itself.

For instance, extensions of Silverman 's teachings will require higher frequencies to accommodate the increased data transmission rates. They will also require a wider bandwidth with a resulting requirement for, either more signaling power, or less attenuation. The signaling power will soon reach practical limitations. Thus, reduced attenuation of the signal will be the only recourse. This implies reduced transmission distances and more repeaters, which, in turn, requires additional frequencies, with attendant cross modulation, frequency management, and supply problems.

On the other hand, extensions of Martins technique will soon encounter stability problems. His transmission channel is, at best, only conditionally stable. Adequate time must be allowed for settling, as all oscillators gradually look into the signal frequency. Thus, it is inherently limited to very slow rates of data transmission.

This invention will provide a data transmission system for use in drilling holes while drilling is in progress, without any necessity for removing the drill string from the hole, or for stopping the drilling in any manner. Furthermore, the system of this invention will be capable of continuous data transmission at increased rates.

Such capabilities of the system of this invention are achieved by the use of repeater stations involving mode transfer transducers which receive signal energy being transferred between these repeater stations by one mode of energy transfer, amplify it, and retransmit this signal energy by a second mode of energy transfer, wherein the receiver of the repeater, while sensitive to the first mode, is insensitive to the second. That is, the first and second modes may be termed orthogonal" in the sense that they are independent modes substantially unaffected by each other.

In addition, where the selected type of transmission modes and/or geologic conditions permit, the system of repeater stations in this invention will be automatically self-stabilizing. This self-stabilization result is achieved by having successive transmissions on each mode reversed in phase from the just previous phase condition on that particular mode. Thus any feedback between nearest neighbor transducers of like modes (remembering that a transducer of another mode will be located between nearest neighbor transducers of a common mode) will be negative and, hence, self-stabilizing.

Where the transmission modes selected and/or the geologic conditions cannot insure that the feedback between transducers of like modes will be negative, mode selective attenuators will be provided at strategic locations to attenuate one specific mode and not the other, and, thus insure that feedback between transducers of like modes is attenuated to the point that the system can remain stable regardless of the phase of the signal fed back by a transducer transmitter of one mode to the transducer receiver of the same mode.

This invention also provides special selective mode attenuators which will attenuate one mode without interference with the other mode being used. Additionally, to provide these attenuators in both active and passive configurations.

In a first exemplary embodiment, the system of this invention comprises mode transducer repeaters which are placed as integral members of the drill string in alternating mode configurations at separation distances predetermined by prior knowledge of the general geology which is expected to be found within the drill hole. In a second embodiment, only the first transducer of each mode is an integral member of the drill string, while other repeater transducers are attached to the drill string in predetermined order, but are so configured that such attachment can be transferred from the drill string to the wall of the drill hole on command from the surface. This allows the drill to penetrate an unknown geologic environment, with the location of the mode transducer repeaters being determined by ongoing evaluation of the signals received at the surface as the drilling progresses.

Thus, this invention relates to a high quality data transmission system for operation in a drill bore hole while drilling is in progress without the necessity of stopping the drilling action. Essentially, it comprises using the drill system and/or the surrounding drilling fluid and/or earth as the transmission medium. It is improved over and therefore differs from other prior art systems in that it uses repeater stations and, at least, two noncoupling or orthogonal modes of transmitting signal energy. The repeater stations are in the form of mode transfer transducers which receive a signal transmitted on one mode, amplify the data thus received, and retransmit it on a second mode of signal energy transfer (to which the receiver of the first mode in the same transducer is insensitive).

In its more general use, the term mode of energy transfer" is used to include any physical energy transfer mechanism capable of being used to transmit data from one point to another. The preferred exemplary method and apparatus will be discussed in detail later. However, examples of non-interacting or orthogonal mode pairs would includes:

1. A mode of electromagnetic energy, and a mode of acoustic energy.

2. Acoustic energy, wherein one mode consists of lateral vibrations introduced into the drill string, and torsional vibrations introduced into the same drill string which, although not completely decoupled, can exist as separate modes over a considerable length of the drill string.

Many other energy transfer methods too numerous to mention also exist as should now be apparent to those in the art. Any one of these can be used to transfer the desired information signal over a short distance where it may be detected, amplified and retransmitted by another mode to which the detector of that same transducer is insensitive.

For the sake of brevity only the preferred exemplary methods and apparatus will be discussed in detail but, it must be remembered that the teachings which make the exemplary method and apparatus new and improved can be applied by analogy to the broader classes of non-interacting modes of signal transmission in general.

In the exemplary method and apparatus discussed below the two non-interferring or orthogonal modes are a lateral current mode and a lateral magnetic mode. These modes have been selected because they are easily excited, have fair to good propagation characteristics, and simple rugged means exist whereby one mode can be coupled to the other.

The exemplary lateral current mode is excited in the drill string by use of a toroid through which the drill string passes. The current is induced by the toroid into the drill string which acts as a one turn winding with the earth providing the return path.

The exemplary lateral magnetic mode is induced by the use of a solenoidal winding around the drill string, wherein the drill string acts as part of the core for the magnetic flux with the earth once again providing the return path for the magnetic flux produced by the solenoid.

These two exemplary modes do not naturally couple one to the other except in so far as secondary field distortions cause a minor coupling. In addition they are truly orthogonal modes where, in the current mode, the current flows in the pipe or drill string and the magnetic field exists at right angles to it in a circumferential pattern around the pipe. In the magnetic mode the exact reverse occurs where the magnetic flux flows in the drill string pipe and the current flows in a circumferential pattern around the pipe. The recognition of these characteristics permit a particularly simple and desirable configuration for the mode transfer transducers utilized in the exemplary embodiment. By simple changes of connections the same transducer unit can be used to perform a number of functions. It can be a current receiver to magnetic transmitter, or a magnetic receiver to current transmitter. It can also be used as either a current or magnetic attenuator where the need exists as will be explained more fully below.

A more complete understanding ofthis invention and appreciation of its improvements and advantages may be obtained from the following detailed description and accompanying drawings, of which:

FIG. 1 is a schematic depiction of an exemplary mode repeater transducer suitable for use in this invention;

FIG. 2 is a diagrammatic view of an exemplary embodiment of this invention actually in use in a drill hole and utilizing a plurality of mode transducers such as shown in FIG. 1;

FIG. 3 is a graph showing relative signal strengths for the various transmission modes along the length of the drill hole shown in FIG. 2;

FIG. 4 depicts an exemplary embodiment ofa passive current mode attenuator for use with this invention;

FIG. 5 is similar to FIG. 4 but in less detail and showing a complete preferred current path;

FIG. 6' is a schematic depiction of an exemplary embodiment of an active current mode attenuator;

FIG. 7 is a schematic depiction of an exemplary embodiment of a combined active current and magnetic mode attenuator; and

FIG. 8 is a block diagram of a further exemplary embodiment of this invention utilizing different signal frequencies as the different transmission modes.

Referring to FIG. 1, a mode repeater transducer 10 is shown as being mounted on a short piece of drill stem or sub 12. The transducer comprises a toroidal transformer l4 surrounding the sub 12 in such a manner that a changing electrical current 16 (shown by solid lines) flowing in the sub 2 and through the toroid 14 will induce a voltage therein which is proportional to the current 16 at the output terminals 18 of the toroid 14. The voltage at output terminals 18 of the toroid 14 is applied to the input of amplifier 20 which then produces an output signal current at 22 proportional to the voltage at terminals 18. This signal current is caused to flow through solenoid winding 24 and return to one of the input terminals 18 thereby providing a common reference, or ground point. In flowing through the turns of solenoid 24, the current from amplifier 20 produces a magnetic flux 26 (shown by a dotted line) proportional to the current 16 and much stronger than, and orthogonal (at right angles) to the circumferential magnetic flux 28 produced directly by the flow of current 16 in sub 12. The changing flux 26 also produces a current 30 which flows circumferentially around sub 2. This current 30 is proportional to, but greatly amplified over, input current 16, but cannot couple back into toroid 14 because its circumferential flow does not encircle the core of toroid l4.

Amplification of the incoming current 16 has thus been achieved without feedback into the input terminals 18 of amplifier 20. It must be noted that if such amplification had been attempted without the utilization of a non-coupling mode, there would have been negligible amplification if the feedback had been negative (in such a direction that the output tended to subtract from the input). And, if the feedback were positive (in such a direction as to add to the input signal) the system would have been unstable, by breaking into oscillation. Accordingly, the use of two noninterferring or noncoupling modes provides a substantially improved transmission system.

It should now be apparent that if the input and output terminals of amplifier 20 were reversed, then exactly the same teaching as applied to the current amplifier, would show that the transducer would act as a flux amplifier, taking flux 26 and amplifying it to produce a much greater flux 28 flowing circumferentially around sub 2 and current 16 flowing laterally in the sub 2. In that flux 28 is parallel to the windings of solenoid 24 cannot couple into the input circuit of amplifier and, again, amplification has been achieved without feedback.

Diagrammatically, the toroid 14 is represented as a block with lines running parallel to the drill stem to indicate the direction of the windings and solenoid 24 is shown with lines running across the drill string again to indicate the direction of windings. Amplifier 20 is shown as a triangle pointed in the direction of amplification and is a relatively standard symbol. Current flow 16 is shown by solid lines with the symbol of a generator to indicate that the current comes from an external source when transducer 10 is used as a current amplifier. Flux 26 is shown by dotted lines. Not shown in the diagram are the necessary details of switching power sources or transformer taps for proper impedance matching. These are details which can be supplied by any competent mechanic in the art.

FIG. 2 gives a diagrammatic view of one of the preferred exemplary configurations of the total system. Once again, conventional details obviously necessary, but peripheral to the understanding of the system, are omitted.

A plurality of mode repeater transducers A through G are mounted on a drill string 50 which is inserted in a bore hole 52. The latter designations are used to indicate differing connections on identical mode repeater transducers and will be explained later. For convenience, the term "mode repeater transducer" will hereafter be referred to by the initials MRT.

MRT A is shown as connected to a data source 54. Since the purpose of this invention is to provide a high quality data transmission channel, the nature of the data source is peripheral to the heart of the invention. The source could represent sensor data output as an aid to drilling, or it could represent data from any or all of a number of sensors for bore hole logging or any other source which generates data to be transmitted to the surface. FIG. 2 shows the amplifier input of MRT A connected to the data source 54 rather than to the output of either the associated toroid or solenoid windings. The amplifier output is connected to the input of the solenoid winding 56 thus the initial mode launched along drill string 50 is magnetic. The magnetic mode containing information from data source 54 propagates along drill string 50 and attenuates with distance as shown in FIG. 3. The plus sign above MRT A indicates the initial time phase of this signal. The distance between MRT A and MRT B is such that only moderate signal attenuation occurs before the signal is picked up by the solenoid coil 58 of MRT B. The MRT B configuration is such that it is connected as a magnetic to current mode transducer as shown. The plus sign indicates that the output current mode from MRT B is still in the same time phase as the input magnetic flux.

The amplified current output of MRT B flows along the drill string 50 and attenuates in both directions as shown in FIG. 3. The spacing between MRT B and MRT C is again such that only moderate attenuation of the current from MRT B occurs prior to reaching MRT C where it is detected, amplified and transferred back into a magnetic mode. Thus in FIG. 2, MRT C is connected as a current to magnetic flux transducer. The minus sign above MRT C now indicates that it is so connected that its output is reversed in time phase with respect to its input. MRT D is again so spaced that only moderate attenuation occurs between MRT C and MRT D and the connection as a magnetic to current mode transducer is the same for MRT B. Its output is in phase with its input and, thus, out of phase with the signal which exists at MRT B. The separation between MRT D and MRT E is variable and increases as the drilling progresses, but it is never allowed to become so great that more than moderate attenuation occurs prior to adding another MRT.

An analysis of the history of the path between MRT E and the next lower MRT will show alternating predominate forms of excitation. The signal existing at MRT E prior to the installation of MRT D would primarily comprise a magnetic flux from MRT C and residual current from MRT B. While FIG. 2 shows MRT E with the toroid 60 and solenoid 62 connected in series (with the output voltages adding), prior to the installation of MRT D, these coils would have been connected with the output voltages subtracting since the two principle components would have been out of phase. The net result would have again been additive voltages. It is obvious that, by proper selection of output connections of the toroids 60 and solenoid 62, the two signals can be made additive regardless of the excitation fields entering MRT E. The output of the amplifier 64 from MRT E is fed to a conventional data receiving and demodulation device 66.

Also shown in FIG. 2 is a special configuration involving two MRTs F and G. These are so connected that they act as attenuators of both magnetic flux and current. The details of this configuration will be discussed later. It is only the function this performs which is important at this point of the teaching.

In the past, the drill 68 has been used as part of the current or magnetic path. From the standpoint of this invention it is a path which, while usable, should preferably be excluded if possible. The drill bit 68 is one of the major sources of noise entering the system and, as such, any currents or flux entering the bit path should be excluded. This is achieved by insertion of attenuators F and G into the path as will be discussed in more detail below.

FIG. 3 represents the various field strengths along the profile of the drill string 50. Here the solid lines are used to represent the magnetic flux mode, while the dotted lines represent the current mode. The displacement of points on these curves represents the relative field strengths with the most distant displacement to the right representing the stronger field; while the plus and minus signs again represent time phase with respect to the initial signal.

The solid lines crossing at the location of MRT B represent the magnetic fields to which the input of MRT B is sensitive. It can be seen that, while the input from MRT A is of a positive phase, that which is fed back from MRT C is of a negative phase or of opposite sign. Thus the feedback of the system is negative and the system is self-stabilizing. The same fact can be observed for the current inputs at MRT C which are again of opposite sign and, thus self-stabilizing.

One familiar with the art of feed back systems will recognize that, if this phasing arrangement were not observed, the system could become unstable and oscillate. Thus, the proper phasing connections of the various mode repeater transducers is an important factor.

The system shown in FIG. 2 is the preferred exemplary configuration where conductivity contrasts, between various layers within the earth are not too great or the frequencies involved are not high (e.g., below approximately 1,000 hertz). However, sharp changes have been observed in the conductivity of the walls of bore holes. Some changes are on the order of one hundred to one, and even approaching one thousand to one, within very short distances. These sharp changes can produce standing waves of reflected energy along the bore hole. While it is true that the signals phase must be continuous and that no sharp changes can occur, phase changes within bore holes have been measured which exceed 180 at higher frequencies, within distances which would be short with respect to desirable separation distances between MRT units. These phase changes raise the possibility of positive feedback which would cause the line of repeaters to oscillate and thus negate the proper functioning of the system.

In such cases where negative feedback cannot be assured, the next preferred approach would be one in which positive feedback either cannot occur or, at least, is unlikely to occur.

It will be observed by reference to FIGS. 2 and 3 that once either mode has been coupled into its respective MRT, the residual flux or current which eventually couples back as feedback serves no useful purpose other than the stabilization thus achieved. In those cases where this feedback could be detrimental, rather than beneficial, the preferred solution of this invention is to block the entry of feedback flux or current by use of strategically located magnetic attenuators thus blocking magnetic flux fields and current attenuators thus blocking feedback currents.

The simplest form of magnetic field attenuator would be the use of a monel metal or a non-magnetic form of stainless steel or other non-magnetic materials as a sub. This special sub section could be inserted between joints of the drill string and centrally located between MRT units to block the flow of magnetic flux along the drill string. As an example: such a unit centrally located between MRT B and MRT C in FIG. 2 would block any magnetic feedback between MRT B and MRT C.

FIG. 4 represents an exemplary passive attenuator for use in the current mode. However, it is not truly an attenuator. It will be seen by the following discussion that it effectively blocks an undesirable current path by establishing a more preferred current path, the currents of which tend to cancel the currents which would otherwise flow in the undesirable current path. It thus acts more in the function of an imperfect switch than as an attenuator. However, for ease of explanation and of visualizing its function it will be referred to as an attenuator.

In FIG. 4, the drill string is shown surrounding by two toroids 23 and 24. Toroids 102 and 104 are connected in series in such a manner that current induced in the drill string 100 by toroid 102 are opposed or cancelled by the currents induced in the drill string 100 by toroid 104. Connected to the drill string 100 and between toroid 102 and toroid 104 are one or more conductors 106 (two are shown) which pass around the outside of toroid 104 and back through the center of toroid 104 adjacent to drill string 100 but insulated therefrom. Conductors 25 then connect to a conducting shell 108 which shell surrounds the drill stem 100 and is insulated therefrom by an insulating shell 110 which may, if desired, be the same insulation as used to insulate conductors 106 from drill string 100. It should be noted that the length of conducting shell 108 and insulator 110 while not critical should be as long as practical consideration permits, and that insulator 110 should extend as far beyond the termination of shell 108 as is reasonably practical.

It will also be noted that since conductors 106 all terminate on the drill string 100 at one end and conductive shell 108 on the other end and all encircle toroid 104 one time in the same direction. They constitute a one turn loop around toroid I04.

In FIG. 4, amplifier 112 is shown as forcing signal current through toroids 102 and 104. Insofar as current induced in the drill string 100 by toroid 102 attempts to flow through toroid 104 it will tend to be cancelled by the currents induced by toroid 104 in drill string 100. However, the current path from the drill string 100 to the surrounding electrode 108 is different. It can be seen that currents induced in conducting loop 106 (which passes through toroid 104 in the opposite direction from the drill string 100) are now in the same direction as the currents which are induced in the drill string 100 by toroid 102 and follow the current path through loop 106 to electrode 108.

It can now be seen that by use of this attenuator scheme it is possible to establish a preferred current path between the drill string 100 and electrode 106 while tending to block the direct current path along the drill string 100 and through both toroids 102 and 104. Although its use is obviously not limited to this location, FIG. 4 shows the attenuator placed adjacent to drill bit 114. This has been done to illustrate two of the desirable features of this device:

I. It tends to block current flow through the drill bit, which, due to resistance and pressure variation, moving parts, and high fluid turbulence can be expected to provide a major source of noise modulation of currents flowing through this path.

2. It allows the location of the communication transmitter (amplifier 112) to be as close to the drill bit as possible, and yet it provides a substantial electrode area 108 which, while physically above the trans-mitting toroids 23 and 24 is electrically below them with respect to the current paths involved. For practical considerations, there is an advantage in having the freedom to locate the information transmitter at this point if so desired.

FIG. shows essentially the same as FIG. 4 except in less current detail. It represents a complete preferred current path, wherein the attenuator unit 150 is of the same configuration as described in the discussion of FIG. 4. While attenuator 152 has not been discussed, those skilled in the art will recognize that the same discussion as that given for the unit 150 is generally applicable, in that the preferred current path is once again established.

The mechanism and desirability of utilizing the output of the toroidal unit 154 to drive the solenoid 156 by amplifier 158 has been established. The preferred current path established by the use of these two units is represented by dotted lines 160.

It will also be recognized by those skilled in the art, that the same teachings as used in the discussion of FIGS. 4 and 5 would apply if the device were to be used as an attenuator of magnetic flux. All that would be necessary would be to replace toroids 102 and 104 by two solenoids connected in the same manner, and to establish that coil 106 and electrode 108 be of material of high magnetic permeability. However, such configuration would not be as efficient for the magnetic mode. Insulator 110 cannot present any degree of magnetic permeability contrast approaching the resistivity contrast achievable between an electrical insulator and the return path when used in the current modes.

FIG. 6 represents a sketch of an active attenuator which, according to its design, may attenuate either electric current or magnetic flux. While the drawing represents a current attenuator and the discussion will pertain to such a current attenuator, it will be recognized that the drawing and discussion will equally well pertain to an active magnetic attenuator, if the word solenoid is used to replace the word toroid, and the alternating current induced in the drill string is referred to as alternating magnetic flux.

Referring to FIG. 6, drill stem 200 is shown surrounding by two toroids 202 and 204. As shown, toroids 202 and 204 are connected so that a voltage applied to the ungrounded terminal of toroid 202 will cause a current to flow in toroid 202 which will induce a current in stem 200, which, on flowing through toroid 204 will produce a voltage of like sign at the ungrounded terminal of toroid 204. The ungrounded terminal of toroid 204 is then connected to the input of amplifier 206, which is of the class of amplifiers known as operational amplifiers. The characteristic of this type of amplifier is such that its amplification, or gain, is so high that it can be considered infinite with respect to the voltages being amplified. Although such infinite amplification is never actually achieved, the discussion is simplified if amplifier 206 is considered to have phase inversion and infinite gain which is common practice in dealing with such amplifiers.

Assume there is an electrical current 208 (indicated by dotted arrows) in drill string 200. Current 208 will cause a voltage to appear at the output of toroid 204..

This voltage will be amplified and inverted in amplifier 206 which will attempt to generate a voltage at its out put which is infinitely greater than, and of opposite sign to, the initial voltage generated at the input by the initial current 208. The infinite voltage from amplifier 206 into toroid 202 will start to generate an infinite current 210 in opposition to current 208, current 210 then becomes an input to be again fed back as was current 208. Thus any current which attempts to flow through toroids 202 and 204, including current caused by the output of amplifier 206, will immediately be cancelled by an equal and opposite current. Ideally, zero alternating current can flow between toroids 202 and 204. Thus, the configuration shown achieves the effect of being an infinite attenuator or insulator, without in any manner jeopardizing the structural integrity of the drill string 200.

Although the discussion refers to ideal components, and such can never be achieved, excellent results can be obtained with far from such ideal components and without infinite gain. In excess of 30 decibels (31.6 to 1) of attenuation has been achieved with configurations such as those shown in FIG. 6. This was achieved with little regard to leakage inductances, stray capacitances, core saturations and nonlinearities which one skilled in the art of transformer design would normally consider. Thus attenuations as high as or even 1000 to one should be easily achievable.

It will now be recognized that the sign conventions used were for purposes of discussion. Any combination of coil connections and inverting or non-inverting amplifiers can be used as long as the net result is that the system attempts to generate a current in the drill string 200 which is much greater and in opposite direction than any current attempting to flow between toroids 202 and 204 on drill string 200.

FIG. 7 is included to show how two of the MRT units discussed under FIG. I could be connected to attenuate both electric current and magneticflux as should now be apparent from the drawing itself. Obviously, either current or magnetic modes could be attenuated independently and individually by disconnecting the amplifier controlling the mode which one does not wish to attenuate.

Returning now to FIGS. 1, 2 and 3, on the basis of the teaching concerning FIG. 6, it is now possible to establish that if the gains of the MRT units A through D are made as high as practical consideration permits, the system will have a high degree of noise immunity.

Adjacent stations are so configured that they are connected by alternate modes of transmission. The first station, such as, C, receives on the same mode as D transmits, but in opposite phase. The broad requirement of the attenuator is met, but only for currents or electromagnetic flux which attempts to impress itself on drill string 50 without having been properly introduced into the data channel. Thus, the system will have an improved degree of immunity to noise introduced along drill string 50.

Where two or more similar devices are discussed as working in conjunction, such as the discussion concerning active attenuation devices, these have been discussed as separate devices as a matter of convenience. Accordingly, it is intended that if several such devices should'be combined on a single core to achieve the same results, this would be within the scope of this invention.

While these teachings have pertained primarily to transmission from a drill hole, they could equally well apply to transmission from the surface into the drill hole, or they could have applied to a two way transmission path, or to transmission between points within the bore hole.

There may be times when one mode, for instance the current mode, may be vastly preferred over the other. These teachings are not changed if the two modes comprise two frequencies, or two combinations of frequen cies, as long as these two frequencies, or combination of frequencies, possess certain inherent characteristics. These characteristics would include:

1. The capability to transform the first frequency, or first group of frequencies, which constitute the first mode, into the second frequency, or second group of frequencies. This is to be achieved by operations performed solely upon the constitutents of the first mode after they have been received at the next MRT unit from the just previous transmitting MRT unit.

2. The capability to transform the second frequency, or second group of frequencies, constituting the second mode back into the first frequency, or first group of frequencies, constituting the first mode. This is to be achieved by operations performed solely upon the constituents of the second mode after they have been received at the next MRT unit from the just previous transmitting MRT unit.

3. The entire operation of transformation from mode one to mode two and back to mode one, to be achieved without loss of the time phase reference of the original mode one transmitted. Of course this includes the same characteristics for the mode two to mode one to mode two transformation.

4. The phase of such transformations being either preset or self-controlled, such that feedback between like modes between adjacent MRT units will be negative and self-stabilizing.

5. The MRT receiver for each mode being insensitive to the other transmitted mode from that MRT.

FIG. 8 presents a flow diagram of one such system, wherein mode one consists ofa single carrier frequency f, and mode two consists of the second harmonic of the mode one frequency 2f. MRT unit 250 receives the mode one frequencyfand, through frequency doubling circuits well known to the art, extracts the second harmonic in such a manner that the data imposed on mode one is not lost. This second harmonic is then retransmitted as mode two 2f. The act of frequency doubling preserves the phase information off. MRT unit 252 receives 2f and, through a frequency divider, divides 2f by 2 and retransmits mode one asf. The act of division by two generates a l80 phase ambiguity. Thusf, as retransmitted by MRT 252 may provide either positive or negative feedback when received by MRT unit 250. If the feedback is positive, the pair of MRT units 250 and 252 will go into uncontrolled signal build up and oscillation. Thus, MRT unit 252 has a stability sensor 254 which detects this uncontrolled build up, or oscillation, and, should such a condition occur, generates a reset pulse to the frequency divider in MRT 252. This reset pulse, indicated by arrow 256 causes the divider to either advance or retard 180 thus causing the output of MRT 252 to generate the condition offas the only stable condition. The feedback, therefor, is either negative, or automatically forced to become negative, and the line becomes self-stabilizing.

Although only a few embodiments of this invention have been described in detail, those skilled in the art will readily appreciate that there are many ways to modify the disclosed system without materially changing the desired functioning or results. Accordingly, all such modifications are intended to be included within the scope of this invention.

What is claimed is:

l. A data transmission system for use with a drill string having a length comprising sections in a bore hole within the earth without requiring electrical conductors running the full length of said sections, said system comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along at least one section length of said bore hole in a first mode of energy transfer at least first and second spaced apart repeater means located along said bore hole at intervals of at least one of said section lengths,

said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals at an amplified energy level along at least one section length of said bore hole in a second mode, said second mode of energy transfer being non-interacting with said first mode,

said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals at an amplified energy level along at least one section length of said bore hole in said first mode, and

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto.

2. A data transmission system as in claim 1 wherein said repeater means include means for inverting the phase of information transmitted on any given mode each time that particular mode is retransmitted.

3. A data transmission system as in claim 1 wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode.

4. A data transmission system as in claim 3 wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode.

5. A data transmission system for use in a bore hole in the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals in a first mode of energy transfer, at least first and second spaced apart repeater means, said first repeater means receiving said first mode from said transmitting means and transmitting said data signals on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, and

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

said transmitting means and said first and second repeater means comprising means for transmitting energy through a drill string in said bore hole and the surrounding earth as a transmission medium.

6. A data transmission system as in claim wherein said first and second repeater means comprise amplification means for providing an amplified output signal on the particular transmission mode involved.

7. A data transmission system as in claim 6 wherein said first energy transfer mode comprises a magnetic flux along said drill string and said second energy transfer mode comprises an electric current along said drill string and wherein said first and second repeater means comprise means for generating said electric current and said magnetic flux in response to received magnetic flux and electric current respectively thereby transducing the first to the second mode and vice versa respectively.

8. A data transmission system as in claim 7 including:

toroidal coils for transmitting and receiving the current or second mode, and

solenoidal coils for transmitting and receiving the magnetic or first mode,

said toroidal and solenoidal coils encompassing at least a portion of said drill string.

9. A data transmission system as in claim 6 wherein said first energy transfer mode and said second energy transfer mode comprise signals at first and second frequencies and wherein said first and second repeater means comprise means for generating said first and said second frequency in response to receiving said second and said first frequency respectively thereby transducing the first to the second mode and vice versa respectively.

10. A data transmission system as in claim 9 wherein said repeater means include means for reversing the phase of each frequency with respect to its previous state each time it is retransmitted.

11. A data transmission system for use in a bore hole in the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals in a first mode of energy transfer, at least first and second spaced apart repeater means, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, and

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

said repeater means including means for inverting the phase of information transmitted on any given mode each time that particular mode is retransmitted, and said transmitting means and said first and second repeater means comprising means for transmitting energy through a drill string in said bore hole and the surrounding earth as a transmission medium.

12. A data transmission system as in claim 11 wherein said first and second repeater means comprise amplification means for providing an amplified output signal on the particular transmission mode involved.

13. A data transmission system as in claim 12 wherein said first energy transfer mode comprises a magnetic flux along said drill string and said second energy transfer mode comprises an electric current along said drill string and wherein said first and second repeater means comprise means for generating said electric current and said magnetic flux in response to received magnetic flux and electric current respectively thereby transducing the first to the second mode and vice versa respectively.

14. A data transmission system as in claim 13 including:

toroidal coils for transmitting and receiving the current or second mode, and

solenoidal coils for transmitting and receiving the magnetic or first mode,

said toroidal and solenoidal coils encompassing at least a portion of said drill string.

15. A data transmission system for use in a bore hole in the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals in a first mode of energy transfer, at least first and second spaced apart repeater means, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and

selective mode attenuators inserted in the transmission path at locations occurring after a particular mode has been received and prior to its retransmission on another mode for attenuation of said particular mode.

16. A data transmission system as in claim 15 wherein said repeater means include means for inverting the phase of information transmitted on any given mode each time that particular mode is retransmitted.

17. A data transmission system as in claim 16 for use in a bore hole in the earth, said transmitting means and said first and second repeater means comprising means for transmitting energy through a drill string in said bore hole and the surrounding earth as a transmission medium.

18. A data transmission system as in claim 17 wherein said first and second repeater means comprise amplification means for providing an amplified output for providing an amplified output signal on the particular transmission mode involved.

19. A data transmission system as in claim 18 wherein said first energy transfer mode comprise a magnetic flux along said drill string and said second energy transfer mode comprises an electric current along said drill string and wherein said first and second repeater means comprise means for generating said electric current and said magnetic flux in response to received magnetic flux and electric currentrespectively thereby transducing the first to the second mode and vice versa respectively.

20. A data transmission system as in claim 19 including:

toroidal coils for transmitting and receiving the current or second mode, and

solenoidal coils for transmitting and receiving the magnetic or first mode,

said toroidal and solenoidal coils encompassing at least a portion of said drill string.

21. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer noninteracting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and

including means whereby said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.

22. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and

wherein said transmitting means and said first and second repeater means include means for causing electromagnetic energy to flow in the drill string in said bore hole and in the surrounding earth.

23. A data transmission system as in claim 22 wherein one of either said first or second modes comprises a magnetic flux along said drill string and the other of either said first or second modes comprises an electric current along said drill string.

24. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole,

said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retrans- 5 mitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and wherein said transmitting means and said first and second repeater means include means for causing acoustical energy to flow in a drill string within said bore hole.

25. A data transmission system as in claim 24 wherein one of said first and second modes is comprised of a torsional vibration along said drill string, and the other of said first and second modes is comprised of a longitudinal vibration along said drill string.

26. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole, said first repeater means recieving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and

wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode,

wherein said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.

27. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmissions of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, and

wherein said transmitting means and said first and second repeater means cause electromagnetic energy to flow in a drill string in said bore hole and in the surrounding earth.

28. A data transmission system as in claim 27 wherein one of said first and second modes comprises a magnetic flux along said drill string and the other of said first and second modes comprises an electric circuit along said drill string.

29. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, and

wherein said transmitting means and said first and second repeater means cause acoustical energy to flow in a drill string within said bore hole.

30. A data transmission system as in claim 29 wherein one of said first and second modes is comprised of a torsional vibration along said drill string and the other of said first and second modes is comprised of a longitudinal vibration along said drill string.

31. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole,

said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode,

said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode,

and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode,

wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode, and

wherein said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.

32. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole,

said first repeater means receiving said first mode from said transmitting means-and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode,

said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode,

wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode, and

wherein said transmitting means and said first and second repeater means cause electromagnetic energy to flow in a drill string in said bore hole and in the surrounding earth.

33. A data transmission system as in claim 32 wherein one of either said first or second modes comprises a magnetic flux along said drill string and the other of either said first or second modes comprises an electric current along said drill string.

34. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer,

at least first and second spaced apart repeater means located along said bore hole,

said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode,

said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode, and

wherein said transmitting means and said first and second repeater means cause acoustical energy to flow in a drill string within said bore hole.

35. A data transmission system as in claim 34 wherein one of said first and second modes is comprised of a torsional vibration along said drill string, and the other of either said first or second modes is comprised of a longitudinal vibration along said drill string.

36. A data transmission system for use in a bore hole within the earth comprising:

data input means for generating data signals,

transmitting means for transmitting said data signals along said bore hole in a first mode of energy tranfer,

at least first and second spaced apart repeater means located along said bore hole,

said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode,

said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode,

receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto,

wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, and

wherein at least one of said first and second repeater means contains a sensing device for detecting the existence of positive feedback through the receiver of the next earlier repeater means and a phase adjustment device for adjusting the phase of the output of said at least one repeater means containing said sensing device in such a manner that the feedback between said at least one repeater means and the receiver of said next earlier repeater means will be negative.

37. A data transmission system as in claim 36 wherein said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.

38. Data transmission system as in claim 36 wherein said transmitting means and said first and second repeater means cause electromagnetic energy to flow in a drill string in said bore hole and in the surrounding earth.

39. A data transmission system as in claim 38 wherein one of said first and second modes comprises a magnetic flux along said drill string and the other of either said first or second modes comprises an electric current along said drill string.

40. A data transmission system as in claim 36 wherein said transmitting means and said first and second repeater means cause acoustical energy to flow in a drill string in said bore hole.

41. A data transmission system as in claim 40 wherein one of said first and second modes is comprised ofa torsional vibration along said drill string and the other of said first and second modes is comprised of a longitudinal vibration along said drill string. 

1. A data transmission system for use with a drill string having a length comprising sections in a bore hole within the earth without requiring electrical conductors running the full length of said sections, said system comprising: data input means for generating data signals, transmitting means for transmitting said data signals along at least one section length of said bore hole in a first mode of energy transfer at least first and second spaced apart repeater means located along said bore hole at intervals of at least one of said section lengths, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals at an amplified energy level along at least one section length of said bore hole in a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals at an amplified energy level along at least one section length of said bore hole in said first mode, and receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto.
 2. A data transmission system as in claim 1 wherein said repeater means include means for inverting the phase of information transmitted on any given mode each time that particular mode is retransmitted.
 3. A data transmission system as in claim 1 wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode.
 4. A data transmission system as in claim 3 wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode.
 5. A data transmission system for use in a bore hole in the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals in a first mode of energy transfer, at least first and second spaced apart repeater means, said first repeater means receiving said first mode from said transmitting means and transmitting said data signals on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, and receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, said transmitting means and said first and second repeater means comprising means for transmitting energy through a drill string in said bore hole and the surrounding earth as a transmission medium.
 6. A data transmission system as in claim 5 wherein said first and second repeater means comprise amplification means for providing an amplified output signal on the particular transmission mode involved.
 7. A data transmission system as in claim 6 wherein said first energy transfer mode comprises a magnetic flux along said drill string and said second energy transfer mode comprises an electric current along said drill string and wherein said first and second repeater means comprise means for generating said electric current and said magnetic flux in response to received magnetIc flux and electric current respectively thereby transducing the first to the second mode and vice versa respectively.
 8. A data transmission system as in claim 7 including: toroidal coils for transmitting and receiving the current or second mode, and solenoidal coils for transmitting and receiving the magnetic or first mode, said toroidal and solenoidal coils encompassing at least a portion of said drill string.
 9. A data transmission system as in claim 6 wherein said first energy transfer mode and said second energy transfer mode comprise signals at first and second frequencies and wherein said first and second repeater means comprise means for generating said first and said second frequency in response to receiving said second and said first frequency respectively thereby transducing the first to the second mode and vice versa respectively.
 10. A data transmission system as in claim 9 wherein said repeater means include means for reversing the phase of each frequency with respect to its previous state each time it is retransmitted.
 11. A data transmission system for use in a bore hole in the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals in a first mode of energy transfer, at least first and second spaced apart repeater means, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, and receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, said repeater means including means for inverting the phase of information transmitted on any given mode each time that particular mode is retransmitted, and said transmitting means and said first and second repeater means comprising means for transmitting energy through a drill string in said bore hole and the surrounding earth as a transmission medium.
 12. A data transmission system as in claim 11 wherein said first and second repeater means comprise amplification means for providing an amplified output signal on the particular transmission mode involved.
 13. A data transmission system as in claim 12 wherein said first energy transfer mode comprises a magnetic flux along said drill string and said second energy transfer mode comprises an electric current along said drill string and wherein said first and second repeater means comprise means for generating said electric current and said magnetic flux in response to received magnetic flux and electric current respectively thereby transducing the first to the second mode and vice versa respectively.
 14. A data transmission system as in claim 13 including: toroidal coils for transmitting and receiving the current or second mode, and solenoidal coils for transmitting and receiving the magnetic or first mode, said toroidal and solenoidal coils encompassing at least a portion of said drill string.
 15. A data transmission system for use in a bore hole in the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals in a first mode of energy transfer, at least first and second spaced apart repeater means, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and selective moDe attenuators inserted in the transmission path at locations occurring after a particular mode has been received and prior to its retransmission on another mode for attenuation of said particular mode.
 16. A data transmission system as in claim 15 wherein said repeater means include means for inverting the phase of information transmitted on any given mode each time that particular mode is retransmitted.
 17. A data transmission system as in claim 16 for use in a bore hole in the earth, said transmitting means and said first and second repeater means comprising means for transmitting energy through a drill string in said bore hole and the surrounding earth as a transmission medium.
 18. A data transmission system as in claim 17 wherein said first and second repeater means comprise amplification means for providing an amplified output for providing an amplified output signal on the particular transmission mode involved.
 19. A data transmission system as in claim 18 wherein said first energy transfer mode comprise a magnetic flux along said drill string and said second energy transfer mode comprises an electric current along said drill string and wherein said first and second repeater means comprise means for generating said electric current and said magnetic flux in response to received magnetic flux and electric current respectively thereby transducing the first to the second mode and vice versa respectively.
 20. A data transmission system as in claim 19 including: toroidal coils for transmitting and receiving the current or second mode, and solenoidal coils for transmitting and receiving the magnetic or first mode, said toroidal and solenoidal coils encompassing at least a portion of said drill string.
 21. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and including means whereby said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.
 22. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and wherein said transmitting means and said first and second repeater means include means for causing electromagnetic energy to flow in the drill string in said bore hole and in the surrounding earth.
 23. A data transmission system as in claim 22 wherein one of either said first or second modes comprises a magnetic flux along said drill string and the other of either said first or second modes comprises an electric current along said Drill string.
 24. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and wherein said transmitting means and said first and second repeater means include means for causing acoustical energy to flow in a drill string within said bore hole.
 25. A data transmission system as in claim 24 wherein one of said first and second modes is comprised of a torsional vibration along said drill string, and the other of said first and second modes is comprised of a longitudinal vibration along said drill string.
 26. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means recieving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, and wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, wherein said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.
 27. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmissions of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, and wherein said transmitting means and said first and second repeater means cause electromagnetic energy to flow in a drill string in said bore hole and in the surrounding earth.
 28. A data transmission system as in claim 27 wherein one of said first and second modes comprises a magnetic flux along said drill string and the other of said first and second modes comprises an electric circuit along said drill string.
 29. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, and wherein said transmitting means and said first and second repeater means cause acoustical energy to flow in a drill string within said bore hole.
 30. A data transmission system as in claim 29 wherein one of said first and second modes is comprised of a torsional vibration along said drill string and the other of said first and second modes is comprised of a longitudinal vibration along said drill string.
 31. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode, and wherein said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.
 32. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the laSt repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode, and wherein said transmitting means and said first and second repeater means cause electromagnetic energy to flow in a drill string in said bore hole and in the surrounding earth.
 33. A data transmission system as in claim 32 wherein one of either said first or second modes comprises a magnetic flux along said drill string and the other of either said first or second modes comprises an electric current along said drill string.
 34. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy transfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissions of said second mode to be phase coherent with the first transmissions of said second mode, wherein said repeater means are arranged to cause the phase of said first mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said first mode, and to cause the phase of said second mode to be reversed on retransmission with respect to the phase of the next earlier transmission of said second mode, and wherein said transmitting means and said first and second repeater means cause acoustical energy to flow in a drill string within said bore hole.
 35. A data transmission system as in claim 34 wherein one of said first and second modes is comprised of a torsional vibration along said drill string, and the other of either said first or second modes is comprised of a longitudinal vibration along said drill string.
 36. A data transmission system for use in a bore hole within the earth comprising: data input means for generating data signals, transmitting means for transmitting said data signals along said bore hole in a first mode of energy tranfer, at least first and second spaced apart repeater means located along said bore hole, said first repeater means receiving said first mode from said transmitting means and retransmitting said data signals along said bore hole on a second mode, said second mode of energy transfer being non-interacting with said first mode, said second repeater means receiving said second mode from said first repeater means and retransmitting said data signals on said first mode, receiving means for receiving and detecting said data signals from at least the last repeater means nearest thereto, wherein said repeater means are arranged to cause all retransmissions of said first mode to be phase coherent with the first transmission of said first mode, and all retransmissiOns of said second mode to be phase coherent with the first transmissions of said second mode, and wherein at least one of said first and second repeater means contains a sensing device for detecting the existence of positive feedback through the receiver of the next earlier repeater means and a phase adjustment device for adjusting the phase of the output of said at least one repeater means containing said sensing device in such a manner that the feedback between said at least one repeater means and the receiver of said next earlier repeater means will be negative.
 37. A data transmission system as in claim 36 wherein said first mode is modulated at a first frequency and said second mode is modulated at a second frequency.
 38. Data transmission system as in claim 36 wherein said transmitting means and said first and second repeater means cause electromagnetic energy to flow in a drill string in said bore hole and in the surrounding earth.
 39. A data transmission system as in claim 38 wherein one of said first and second modes comprises a magnetic flux along said drill string and the other of either said first or second modes comprises an electric current along said drill string.
 40. A data transmission system as in claim 36 wherein said transmitting means and said first and second repeater means cause acoustical energy to flow in a drill string in said bore hole.
 41. A data transmission system as in claim 40 wherein one of said first and second modes is comprised of a torsional vibration along said drill string and the other of said first and second modes is comprised of a longitudinal vibration along said drill string. 